The main purpose of this proposal is to develop a novel ion source for biological mass spectrometry (MS). It will increase the ionization efficiency of biomolecules in the matrix-assisted laser desorption/ionization (MALDI) technique by post-ionizing neutral molecules desorbed into a gas phase in the MALDI process. Post-ionization will be achieved via proton transfer reactions with protonated reagent gas ions produced in the chemical ionization (Cl) source. This MALDI-CI source will be interfaced to an orthogonal acceleration time-of-flight (TOP) mass spectrometer using a quadrupole ion guide. Decoupling of the proposed ion source from the TOP mass analyzer will allow operating it at elevated pressures (approximately 1 Torr), thus increasing the effectiveness of the chemical post-ionization. By using reagent gases with different proton affinities we can induce and control the in-source fragmentation of peptide ions. Together with the increased ionization efficiency this will make mass spectrometric identification of proteins and their posttranslational modifications more sensitive and reliable.